London, England (CNN) -- As the knockout stages of the 2010 FIFA World Cup begin, it's not only the teams dropping out of the tournament.

Two new stadiums in South Africa have also finished their work.

Having hosted just four matches each, the Peter Mokaba Stadium in Polokwane and the Mbombela Stadium in Nelspruit now face an uncertain future as organizers attempt to secure a financially viable future for the 40,000-seat arenas.

Should they fail, the stadiums, built at a cost of around 1.3 billion Rand ($172 million), risk becoming costly white elephants in areas already blighted by poverty.

Not that South Africa would be the first nation to spend millions of dollars on stadiums for a major event only to find the facilities fall into disuse.

Many of the venues built for the 2004 Olympics in Athens, Greece, have fallen into a state of disuse and disrepair.

But white elephant stadiums could be an endangered species as architects, event organizers and governments put sustainability at the heart of their plans.

Architect John Barrow, a senior principal at global design practice Populous, believes construction of sporting arenas could be radically different in the future.

At a recent lecture delivered at the Australian Embassy in London, Barrow described a vision of a "stadium in a box," -- flexible, modular arenas, which could be tailored to the needs of an event.

Today, nobody's interested in having stadiums which are too big.--John Barrow, Populous

The 2012 Olympic stadium in London, which Populous designed, conforms to this modular model.

The core structure is a 25,000 seat arena, with modular seating taking the capacity up to 80,000 for the duration of the games.

The other option, Barrow says, could be to build modular stadiums from scratch using prefabricated techniques. Once an event is over it could be taken down and transported to another event somewhere else in the world.

"Personally, I think that's where the future is for these events. The way ahead is not to build huge monuments but to do something which is far more important for the legacy and community later," Barrow said.

Populous, who also helped co-design the Soccer City Stadium in Johannesburg, are experts in stadium design, having helped design the roof at Wimbledon's Center Court.

For them, the breakthrough with sustainability, says Barrow, came at the 2000 Summer Olympics in Sydney.

Their design allowed for a 30,000 reduction in capacity -- 110,000 during the games to 80,000 afterwards -- and also incorporated sustainable features like rainwater harvesting, natural convection and a lightweight polycarbonate roof.

Today, Barrow said, Populous look at every single material and system from a sustainability point of view.

"Everything has to be lightweight, recyclable or locally sourced," Barrow said.

"Some of the things we are doing now we couldn't have done five years ago. The Wimbledon roof fabric, for example, wasn't available five years ago," he said.

"Five to ten years ago we weren't geared up for sustainability issues, but now we are."

Barrow thinks modular designs could also make it easier for "emerging nations" to tender bids for global sporting events.

"It could shorten the construction timetable of traditionally built stadium by at least 30 percent," Barrow said.

Although building material costs remain largely the same, Barrow says, the construction costs would be greatly reduced.

The stadiums at this year's FIFA World Cup has been a mixture of old and new design.

On the one hand, there are the stadiums at Polokwane and Nelspruit.

"I'm concerned about the legacy of those stadia," Barrow said, "however the South African government is determined to make sure there is a community legacy."

But modular seating has been incorporated at the stadiums in Cape Town and Durban, temporarily raising the capacity in both by 13,000 seats.

Rainer Quenzer, Head of Planning at Nussli -- a Swiss company that developed the modular concepts for the stadiums in Durban and Cape Town -- estimates that fully modular stadiums cost around half that of a permanent structures, and could stop governments being saddled with white elephant arenas.

"To build a 20-30,000-seat stadium extend for an event, and then shrink it afterwards might be the way to go in the future," Quenzer told CNN.

Nussli have recently finished the construction of a temporary stadium (capacity 27,500) for the BC Lions football team in Vancouver, Canada, and are currently building a 50,000 venue for a national wrestling event in Switzerland.

They have even developed a concept for a 70,000-seat modular arena.

"This approach can be the future and this is our vision, but it will take another generation," Quenzer said.

The strong legacy element of London's successful bid to host the 2012 games clearly impressed the International Olympic Committee back in 2005 when they made their decision.

And John Barrow says FIFA and UEFA are starting to pick up on this too.

"Today, nobody's interested in having stadiums which are too big," he said.

"You need to build in huge amounts of flexibility. Start small and keep it simple."